In a lot of ways I see the Wii as being heavily related to the original Gameboy ...
At the time of its release the Gameboy was unlike anything the industry had really seen before; certainly, there were portable games before (and even Nintendo had previously seen success with the Game and Watch) but the Gameboy was the first handheld game system as we know it today.
From (pretty much) day one the Gameboy faced countless more powerful systems with higher resolution and colour screens, but the Gameboy survived and Nintendo thrived. Nintendo's amazingly polished, creative game development really outmatched all of the processing power their competition threw at them.
Personally, I believe that Nintendo is as creative as they have ever been (their polish is still very good, although more developers are able to match it today) and I wouldn't be surprised if they could pull off a Wii Sports/Pokemon/Nintendogs/Brain Training craze every year for the next 5 (or so) years on the Wii. If they're successful with this they will (indirectly) show developers how to take advantage of the Wii, which will also result in many good third party Wii games.
